Enhanced cutter profile for fixed cutter drill bits

ABSTRACT

In one aspect, a cutting structure for a drill bit includes cutters arranged to form a cutter profile. Substantially all of the cutters in a shoulder section of the cutter profile are positioned at a profile angle of at most 65°. In another aspect, substantially all of the shoulder cutters are arranged so that the longitudinal force exerted by each shoulder cutter onto an earthen formation is at least 25 percent of the normal force exerted by the shoulder cutter onto the earthen formation.

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is claimed from U.S. Provisional Application No. 61/265,030filed on Nov. 30, 2009.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to fixed cutter drill bits used fordrilling wellbores in subsurface earthen formations. More particularly,the present invention relates to the arrangement of cutting elements ondrill bits used to shear earthen formations in so-called “fixed cutter”drill bits.

2. Background Art

Fixed cutter wellbore drill bits known in the art generally includecutting elements arranged in such a way that if the bit were rotatedabout its longitudinal (rotational center) axis, the edges of all of thecutters would define a “cutter profile” or “bottom hole pattern.” Whenviewed as a section through the longitudinal axis, a two dimensional(2D) cutter profile drawn through the tips of all of the cutterstypically includes a linear ‘cone’ shaped section toward or proximatethe rotational center of the bit, arcuate ‘nose’ and ‘shoulder’sections, and a linear ‘gage’ section substantially tangent to thearcuate shoulder section and substantially parallel to the bit'slongitudinal axis. Such profile shape is the result of decades ofdevelopment in the fixed cutter drill bit industry, and is so shaped fora variety of reasons, one of the most important being the ability toincrease the density of cutter elements with increasing radial (lateral)displacement from the longitudinal axis along the profile.

Generally, fixed cutter bits that are used to drill “hard” earthenformations have a longer bit profile than bits used to drill softerformations. Longer bit profiles allow the placement of additionalcutting elements compared to shorter bit profiles, and it is generallydesired to have more cutting elements in the profile when drillingharder formations. The cutters on a fixed cutter drill bit are arrangedto create a smooth transition between the different sections of the bitprofile, and are also generally arranged in such a way that the ‘profileangle’ of the cutters increases with increasing radius from 0° at thenose position to a maximum value around 90° at the ‘gage point’. The‘profile angle’ of a cutter is defined as the angle between a line drawnthrough the center of the cutter, perpendicular to the 2D bit profile,and the longitudinal axis of the drill bit. A table of cutter locationsincluding profile angles can be found in U.S. Pat. No. 5,678,644 issuedto Fielder. The ‘gage point’ is defined as the point of the furthestradial extent of the bit profile, at the closest longitudinal locationto the nose of the drill bit. Examples of prior art bit profiles can beobserved in many patents including U.S. Pat. No. 6,575,256 issued toDoster.

Fixed cutter, polycrystalline diamond compact (PDC) cutting element bitswere first used in the field of drilling earth formations in the 1970's.For many years, fixed cutter drill bits were primarily used for drillinglow-strength formations, as the cutters could not withstand the forcescreated when drilling harder formations. More recently, however, fixedcutter bit design and cutter technology have improved to such an extentthat enables fixed cutter bits to drill many harder formations moreeffectively than other types of drill bits. However, there are stillmany formations that are too hard or too abrasive to be effectivelydrilled with fixed cutter bits, and these formations represent the mostexpensive drilling in the industry as penetration rates are very low,and drill bit life is very short.

Many attempts have been made to further increase the wear resistance offixed cutter bits when drilling hard and abrasive formations, but thoseattempts have primarily focused on either adding additional diamond tothe drill bit (refer to U.S. Pat. No. 7,594,554, the “554 patent”), oron developing new types of cutters that can better withstand theconditions experienced when drilling harder formations (refer to U.S.Pat. No. 6,544,308 issued to Griffin et al.). So called ‘double-row’fixed cutter layouts, such as described in the '554 patent, are nowcommonplace in state of the art fixed cutter PDC drill bits due to theirsuccess in extending drill bit life in hard and abrasive formations.This improvement in drill bit life is simply a result of adding morediamond to the drill bit to increase overall wear resistance. However,in applications where the formation is even more hard and abrasive thanthose drilled using a bit as described in the '554 patent, evendouble-row fixed cutter bits still experience very short life due to therapid deterioration of the PDC cutters.

As is well known in the art, the wear life of PDC cutters is greatlyaffected by the temperature the cutter experiences while drilling, andby the depth of cut achieved by the cutter while drilling. It is welldocumented in the literature that, in a given formation, when a fixedcutter bit drills with a larger depth of cut, the wear life of thecutters increases dramatically (ref. IADC/SPE paper no. 39306 authoredby Sinor et al.). This effect is described in the literature as‘shearing’ versus ‘scraping’. Fixed cutter diamond drill bits have beenused to drill earthen formations for over 100 years, but until theintroduction of the PDC cutter in the 1970's, drilling efficiency wasgenerally poor due to the fact that the previous fixed cutter bitsgenerally drilled by scraping or grinding rather than shearing theformation. With the introduction of the PDC cutter, also called a ‘shearcutter’, drilling efficiency was dramatically improved, allowingboreholes to be drilled faster and more economically than ever before.However, in so-called “ultra-hard” formations it can be very difficultto achieve a “shearing” depth of cut with a fixed cutter drill bit.Generally, when attempting to drill ultra-hard and abrasive formationswith fixed cutter drill bits, in order to achieve an effective bit life,fixed cutter bits are designed with a large plurality of cutters, whichis achieved through a combination of high blade count, long cutterprofile, and double-row layouts. In order to achieve shear with such aheavy-set fixed cutter bit, it is necessary to apply a very large weighton bit (WOB). However, when high WOB is used, there is an increasedtendency to cause mechanical failure of the cutters. Also, becausefrictional heating of the cutters is directly proportional to WOB, usinga high WOB increases the temperature of the cutters, which furthercontributes to cutter deterioration. As a result, fixed cutter bits aregenerally not effective for drilling in ultra-hard and abrasiveformations.

Accordingly, there is a need for fixed cutter drill bits that exhibitincreased durability when drilling hard and abrasive formations. A fixedcutter drill bit with enhanced durability, using existing materials,will allow the bit to drill longer sections in hard and abrasiveformations.

SUMMARY OF THE INVENTION

A cutting structure for a fixed cutter drill bit according to one aspectof the invention includes cutters arranged to form a cutter profile.Substantially all of the cutters in a shoulder section of the cutterprofile are positioned at a profile angle of at most 65 degrees.

A cutting structure for a fixed cutter drill bit according to anotheraspect of the invention includes cutters arranged to form a cutterprofile. Substantially all of the cutters in a shoulder section of thecutter profile are arranged such that the longitudinal force exerted onan earthen formation by each cutter in the shoulder section is at least25 percent of a normal force exerted by that cutter on the earthenformation.

A fixed cutter drill bit according to another aspect of the inventionincludes a bit body having a plurality of blades extending laterallyaway from a longitudinal center of the bit body. A plurality of cuttersaffixed to the at least one of the plurality of blades define a cutterprofile having a shoulder section. Substantially all of the cutters inthe shoulder section of the profile are positioned to at least one of(a) a profile angle of at most 65 degrees and (b) such that alongitudinal force exerted on an earthen formation by each cutter is atleast 25 percent of a normal force exerted by such cutter on theformation.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an oblique view of an example fixed cutter drill bit.

FIG. 1 shows a prior art cutter profile.

FIG. 2 shows an example profile according to the present invention.

FIG. 3 shows an exemplary cutter oriented at a profile angle α, actingon a formation.

DETAILED DESCRIPTION

FIG. 1 shows a prior art cutter profile 10 having a cone section 12, anose section 14, a shoulder section 16, a gage point 18, and a gagesection 20. On the cutter profile 10 is an example cutter of circularcross-section 22, positioned in the shoulder section 16 at a profileangle α with respect to the longitudinal axis 24 of the drill bit. It iscommon for cutters in the shoulder section 16 on prior art fixed cutterbits to be arranged with a maximum profile angle of between 70-90degrees The normal force (F_(N)) that the cutter exerts on the formationis created by the axial loading or weight on bit (“WOB”), which is theforce applied by the drill string along the longitudinal drill bit axis.As shown in FIG. 3, the normal force can be further resolved into itscomponents which include a longitudinal force (F_(L)) and a radial force(F_(R)), both of which components are resultants of the normal forceF_(N) and the profile angle. The longitudinal component of the normalforce defines a cutter's ability to remove formation that islongitudinally adjacent to (ahead of) the cutter, thus defining a fixedcutter drill bit's ability to penetrate earthen formations. Therefore,the larger the profile angle of a cutter, the smaller the correspondinglongitudinal force on the cutter, and the lesser the ability topenetrate the formations longitudinally forward of the cutter. Thecutters in the shoulder section 16 of the cutter profile 10 that are atthe largest radial extent from the bit axis, and therefore also orientedat the highest profile angle, therefore exert the least amount oflongitudinal force on the formation.

Having explained a prior art cutter profile, various implementations ofa cutter profile according to the invention and a drill bit madetherewith will now be explained. A drill bit made using a cutter profileaccording to any of the examples explained herein below is shown ingeneralized form in FIG. 1A, wherein the drill bit 11 includes a bitbody 13 of a type well known in the art for affixing thereto a pluralityof PDC cutters 15. Any or all of the cutters shown in the example bit 11in FIG. 1A may be arranged according to any of the example profilesexplained herein, and it is to be clearly understood that the bit bodyand cutter configuration shown in FIG. 1A are only meant to serve as anexample of a drill bit made using cutter profiles according to theinvention. Accordingly, the bit and cutting structure and numbers ofcutters on the bit as shown in FIG. 1A are in no way intended to limitthe scope of the present invention. Generally, the cutter profile isdefined by the edges of cutters arranged on one or more blades 17, whichextend laterally away from a center or longitudinal axis of the bit body13. The invention relates to the profile angle of certain of the cutters15 disposed in a selected portion of the blade (and thus the profile).

Now referring to FIG. 2, in an example embodiment of the presentinvention, the cutter profile 10 includes a cone section 12, a nosesection 14, a shoulder section 16, a gage point 18, a gage section 20,and an example cutter 22 positioned in the shoulder section 16 at aprofile angle β with respect to the longitudinal axis 24 of the drillbit. The foregoing profile components are similar qualitatively to thosein the prior art profile of FIG. 1. Unlike the prior art cutter profile,however, a cutter profile according to the present invention includes avery sharp transition at the gage point 18 and is arranged such that thecutters in the shoulder section 16 remain at a relatively low profileangle. In one embodiment of the present invention, the profile angle maybe at most 65 degrees. In other embodiments, the profile angle may be atmost 45 degrees. In still other embodiments, the profile angle may be atmost 30 degrees. Because the cutters in the shoulder section 16 areoriented on the cutter profile 10 at a low profile angle as contrastedto those of prior art fixed cutter bits, the longitudinal force exertedby those cutters in the shoulder section 16 on the formation is muchlarger than that of the cutters in the shoulder section on prior artfixed cutter bits. As a result of the larger longitudinal forceexhibited by the cutters in the shoulder section 16 of a bit accordingto the present invention, it is possible to achieve shear cutting of theformations even in ultra-hard formations with a relatively low axialforce or weight on bit (WOB). Finally, because the WOB can be kept at arelatively low value, frictional heating on the cutters is lessened ascontrasted with prior art profile fixed cutter bits, thus furtherprotecting the cutters from thermally induced wear. FIG. 3 shows anexemplary cutter 22 oriented at a profile angle α, acting on aformation, represented by line 26, and showing the forces that resultfrom this interaction. The normal force, F_(N), is parallel to a linethrough the center of the cutter 22, and is perpendicular to the cutterprofile. The longitudinal force, F_(L), is the component of the normalforce acting in a direction parallel to the longitudinal axis of thedrill bit. The radial force, F_(R), is the component of the normal forceacting in a directional normal to the longitudinal axis of the drillbit. As can be observed in the example of FIG. 3, because the totaldriving force for fixed cutter drill bits is the WOB, and because thatforce is directed to the cutting structure along the bit axis, thelarger the profile angle, α, of a cutter, the lower the longitudinalforce acting on the formation. When drilling ultra-hard formations, avery large longitudinal force is required to allow a large enough depthof cut to shear the formation. Because prior art drill bits are designedwith long profiles that have the shoulder cutters arranged at very highprofile angles, those cutters do not exert sufficient longitudinal forceto penetrate the formation, and therefore only cut the formation byscraping, which is a very inefficient mechanism to cut or fail abrasiveformations and results in very high temperatures being generated. It isprimarily for this reason that cutters on the outer shoulder of priorart fixed cutter drill bits experience rapid wear when drillingultra-hard abrasive formations.

In some embodiments of a drill bit according to the invention, theprofile angle of the cutters in the shoulder section 16 is such that thelongitudinal force exerted on the formation by each cutter in theshoulder section is at least 25% of the normal force exerted by saidcutter on the formation. In other embodiments, the cutter profile angleof the cutters in the shoulder section is selected such that thelongitudinal force exerted by such cutter on the formation is at least50% of the normal force exerted by such cutters on the formation. Instill other embodiments, the cutter profile angle of the cutters in theshoulder section is selected such that the longitudinal force exerted byeach cutter on the formation is at least 75% of the normal force exertedby such cutter on the formation.

In another embodiment the bit is designed and built with backup gagecutters positioned at the gage point, where the cutter profile meets thegage. The radially outermost cutter before the gage point is positionedso that its edge is tangent to the gage of the bit. All gage cutters maybe positioned with sufficient side rake to provide clearance between thecarbide substrate of the PDC cutter and the gage of the bit where itwould contact formation. Additionally, if consistent with other drillingparameters and objectives, cutters with a 16 millimeter diameter may beused as the primary cutter size, but backup gage cutters may be 13millimeter diameter. While the present invention is described in termsof polycrystalline diamond compact (“PDC”) cutter drill bits andcircular cross-section cutters, the invention is equally applicable toall types and shapes of cutters used in fixed cutter drill bits.Accordingly, the material composition and the cross sectional shape ofthe cutter used in any embodiment is not a limitation on the scope ofthe present invention.

Further, while the invention has been described with respect to alimited number of embodiments, those skilled in the art, having benefitof this disclosure, will appreciate that other embodiments can bedevised which do not depart from the scope of the invention as disclosedherein. Accordingly, the scope of the invention should be limited onlyby the attached claims.

1. A cutting structure for a fixed cutter drill bit, comprising: cuttersarranged to form a cutter profile on part of a bit body; and whereinsubstantially all of the cutters in a shoulder section of the cutterprofile are positioned at a profile angle of at most 65 degrees.
 2. Thecutting structure of claim 1 wherein substantially all of the cutters inthe shoulder section are positioned at a profile angle of at most 45degrees.
 3. The cutting structure of claim 1 wherein substantially allof the cutters in the shoulder section are positioned at a profile angleof at most 30 degrees.
 4. A cutting structure for a fixed cutter drillbit, comprising: cutters arranged on part of a bit body to form a cutterprofile; and wherein substantially all of the cutters in a shouldersection of the cutter profile are positioned such that a longitudinalforce exerted on an earthen formation by each cutter is at least 25percent of a normal force exerted by that cutter on the earthenformation.
 5. The cutting structure of claim 4 wherein each cutter inthe shoulder section is positioned such that the longitudinal forceexerted on the earthen formation by that cutter is at least 50 percentof the normal force exerted by that cutter on the earthen formation. 6.The cutting structure of claim 4 wherein each cutter in the shouldersection is positioned such that the longitudinal force exerted on theearthen formation by that cutter is at least 75 percent of the normalforce exerted by that cutter on the earthen formation.
 7. A fixed cutterdrill bit comprising: a bit body having a plurality of blades extendinglaterally away from a longitudinal center of the bit body; a pluralityof cutters affixed to at least one of the plurality of blades anddefining a cutter profile having a shoulder section; and whereinsubstantially all of the cutters in the shoulder section of the cutterprofile are positioned to at least one of (a) having a profile angle ofat most 65 degrees and (b) such that a longitudinal force exerted on anearthen formation by each cutter is at least 25 percent of a normalforce exerted by such cutter on the earthen formation.
 8. The bit ofclaim 7 wherein substantially all of the cutters in the shoulder sectionare positioned to at least one of (a) having a profile angle of at most45 degrees and (b) such that the longitudinal force exerted on anearthen formation by each cutter is at least 50 percent of the normalforce exerted by such cutter on the earthen formation.
 9. The bit ofclaim 7 wherein substantially all of the cutters in the shoulder sectionare positioned to at least one of (a) having a profile angle of at most30 degrees and (b) such that the longitudinal force exerted on anearthen formation by each cutter is at least 75 percent of the normalforce exerted by such cutter on the earthen formation.
 10. The bit ofclaim 7 wherein the cutters comprise polycrystalline diamond compacts.11. The bit of claim 8 wherein the cutters comprise polycrystallinediamond compacts.
 12. The bit of claim 9 wherein the cutters comprisepolycrystalline diamond compacts.